107 Chapter -5 DEVELOPMENT OF A GEOSTATIONARY BEACON SYSTEM FOR TEC MEASUREMENT AND SIMULATION OF ORTHOGONAL CODED SPREAD SPECTRUM SYSTEM This chapter gives details on the design and development of a coherent beacon receiver system suitable for reception of amplitude modulated beacon signals from the Indian geostationary beacon payload CRABEX. Details of simulation of an orthogonal coded spread spectrum system is given which can be used to deal with the loss of lock observed in the above system. 5.1 Introduction Communications have come to rely heavily on ionospheric radio in spite of the sometimes unpredictable behavior of the ionosphere as a transmission medium. Far from being the static reflector of radio waves that the communicator would desire, the ionosphere continuously and sometimes suddenly, undergoes structural changes on almost all scales of size and time. Such changes upset the often delicate balance of operational parameters which must work together to optimize communications. This can be attributed to fading, low signal strength or even loss of signal. The properties of the ionosphere which govern these changes have been the object of research since the earliest days of radio. In the previous chapters, the importance of understanding and characterizing the continuous variation of ionosphere has been addressed. As seen, there are some LEO beacon satellites available as of now, to initiate a study in this direction for understanding the behavior of ionospheric electron density spatially by the method of ionospheric tomography, with simultaneous data from a chain of receiving stations. But, with a coherent LEO beacon satellite, the data availability is sparse. The advent of geostationary beacon satellites for ionospheric studies has made possible measurement of long continuous records of total electron content for many
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107
Chapter -5
DEVELOPMENT OF A GEOSTATIONARY BEACON SYSTEM FOR TEC MEASUREMENT AND SIMULATION OF
ORTHOGONAL CODED SPREAD SPECTRUM SYSTEM
This chapter gives details on the design and development of a coherent
beacon receiver system suitable for reception of amplitude modulated beacon
signals from the Indian geostationary beacon payload CRABEX. Details of
simulation of an orthogonal coded spread spectrum system is given which can be
used to deal with the loss of lock observed in the above system.
5.1 Introduction
Communications have come to rely heavily on ionospheric radio in spite of the
sometimes unpredictable behavior of the ionosphere as a transmission medium. Far
from being the static reflector of radio waves that the communicator would desire,
the ionosphere continuously and sometimes suddenly, undergoes structural changes
on almost all scales of size and time. Such changes upset the often delicate balance
of operational parameters which must work together to optimize communications.
This can be attributed to fading, low signal strength or even loss of signal. The
properties of the ionosphere which govern these changes have been the object of
research since the earliest days of radio.
In the previous chapters, the importance of understanding and characterizing the
continuous variation of ionosphere has been addressed. As seen, there are some LEO
beacon satellites available as of now, to initiate a study in this direction for
understanding the behavior of ionospheric electron density spatially by the method
of ionospheric tomography, with simultaneous data from a chain of receiving
stations. But, with a coherent LEO beacon satellite, the data availability is sparse.
The advent of geostationary beacon satellites for ionospheric studies has made
possible measurement of long continuous records of total electron content for many
Chapter-5
108
fixed locations on Earth, helping to study the ionosphere by using multiple
frequencies, as explained by Garriott and Little [1960].
A geostationary satellite because of its orbit appears stationary above any particular
place on Earth. Hence in order to track such a satellite, detailed ephemeris is not
required. Once the satellite is launched and positioned, the location can be known
and a ground station antenna can be pointed towards this location. Also, unlike a
LEOS beacon receiver antenna, which has a wide beamwidth to cover from horizon
to horizon, the antennas for receiving geostationary beacon signals have a narrower
beamwidth to point to the satellite. This will also result in a higher gain for the
ground receiving system.
5.2 Need for a geostationary beacon
Geostationary satellites allow the observation of changes in the ionospheric electron
content under nearly constant geophysical conditions. From Low Earth orbiting
satellites one can derive primarily spatial changes of electron content, since the time
for a scan provided by a satellite pass is short compared with timescales typical for
ionospheric processes (except scintillations) as mentioned by Pulinets et al [1996].
Polar orbiting satellites also provide a scan in latitude for nearly constant local time
if the observed electron content is referred to a ionospheric point in a given height.
The disadvantage of using LEO satellites for ionospheric studies is the bad time
resolution even when several satellites can be observed; the time duration from one
useful pass to the next is of irregular interval. On the other hand, geostationary
satellites provide no spatial resolution at all when only one observing station is used.
Hence a combination of data from low earth orbiting and geostationary satellites
could be used to override these advantages, as addressed by Leitinger [1972].
The orbital height of geostationary satellite is 36,000 km. By using multiple
coherent frequencies with frequency ranges above 130 MHz as the beacon signals,
TEC can be derived in different ways. The popular methods of TEC measurements
with a geostationary beacon include Faraday rotation (FR), Differential Doppler
(DD) and Modulation phase delay (MPD). The method proposed by Smith [1971]
also addresses the removal of n ambiguity, which is one major constraint with
Chapter-5
109
measurements from LEO beacon systems. It has been shown from several earlier
works like Ramarao et al [2004], that above 5000 km the upper atmosphere does
not contribute measurably to the Faraday rotation angle. This is due to the decrease
in the weighting function of the Earth's magnetic field and also a decrease in
electron content with increasing altitude. The Differential Doppler is not affected by
the above mentioned weighting function and therefore is measurable along
the total range of upto 36,000 km. The extraction of TEC from measurement of
MPD throws light on the coarse variation of TEC, which addresses the nπ ambiguity
with the TEC measured with DD technique.
5.3 Genesis of geostationary beacon systems
The first geostationary beacon reported to be used widely for ionospheric studies is
from the ATS-6 experiment. This satellite, launched in 1974, carried a multi-
frequency radio beacon as detailed in Davies et al [1975], Grubb [1972]. The
reception of ATS-6 from a number of ground receiving stations permitted
continuous monitoring of the integrated electron content in the ionosphere as
reported by a series of papers described in the Proceedings of the Satellite beacon
group symposium of COSPAR, 1976.
This geosynchronous satellite ATS-6 placed in orbit over 94º West meridian in May
'74 was relocated over 35º East meridian for a period of one year, from August 1975
to July '76. From this location, the satellite was visible for observations by receiving
stations in India. One of the receiver locations stationed at VSSC, Trivandrum aimed
at scintillation studies. The detail of development of the receiver system for ATS-6
is provided in VSSC Technical Report [1977]. Thus the ATS-6 satellite provided for
the first time to Indian experimenters, highly phase coherent radio beacon
transmissions from a stationary source, thus making possible, study of the
ionospheric and plasmaspheric total electron content, and the phase and amplitude
scintillations over a wide range of frequencies, as detailed in various research works
from SPL, Trivandrum and PRL, Ahmedabad.
There has been a lull before the launch of the next geostationary beacon, probably
because of the huge cost involved with launch. The next known GSAT coherent
Chapter-5
110
beacon payload was flown in the Indian mission of GSAT-2 in 2003. The Coherent
RAdio Beacon EXperiment (CRABEX) payload designed and developed by VSSC
was one of the scientific experiments of opportunities onboard the second Indian
geostationary mission launched from Sriharikota (SHAR). This satellite was positioned
at an angle of 55° elevation and 263° azimuth from true North. This formed the second
phase of the CRABEX national project, aimed to carry out detailed studies on the low
latitude ionosphere. (The detail of the first phase has been covered in Chapter 3). Here,
the investigation is carried out for studying the propagation characteristics of coherent
signals at VHF and UHF transmitted by the onboard beacon in GSAT-II payload.
5.4 The Coherent Radio Beacon Experiment (CRABEX) national
project - Phase II
The onboard beacon of the CRABEX payload transmits four coherent frequencies, two
in VHF and two in UHF, with linear polarisation. The frequencies chosen are 400.032
MHz, 399.03192 MHz, 150.012 MHz and 149.01192 MHz. These signals traverse the
atmosphere and reach the ground station. In the ground receiver system, this is received
as two circular polarisations (LCP & RCP). The receiver processes the RCP and LCP
signals separately. The RCP chain separates out the incoming frequencies to generate a
phase coherent IF from the 400MHz, which forms the reference signal used for phase
comparison with other frequencies. The ionospheric parameters derived from this data
are Differential Doppler (DD), Modulation Phase Delay (MPD), Faraday Rotation
(FR) and Scintillation (amplitude and phase). Out of this, the initial three parameters
are related to TEC, wherein the first two together give the Integrated Electron Content
(IEC) from the satellite to ground and the third one gives the TEC from ground to ~
2000 km. In order to measure these, the receiver generates the following outputs.
The phase difference between RCP and LCP of VHF carrier, which is twice
the Faraday rotation. The Faraday rotation is the angular rotation of the plane
of polarisation undergone by the VHF carrier.
The differential phase between the VHF and UHF carrier which is known as
Differential Doppler.
Chapter-5
111
The phase difference between the low frequency coherent CW modulation of
~ 1MHz on the VHF and UHF, called as Modulation Phase Delay. This is
required to unwrap the 2nπ ambiguity of the Differential Doppler.
Signal strengths of all the four frequencies which can be correlated to
amplitude and phase scintillations at the frequencies.
There have been very few geostationary beacon satellites in the past to help conduct
long term ionospheric research and the present Coherent Radio Beacon Experiment
project offers a unique opportunity in every sense.
5.4.1 Scientific objectives of CRABEX Phase II
The major scientific objectives for the GSAT phase of CRABEX project can be
listed as:
Measurement and comparison of integrated total electron content using three
different techniques – viz, Differential Doppler, Modulation phase delay and
Faraday rotation.
Determination of plasmaspheric electron content (PEC), which is the
difference between the TEC measured by Differential Doppler and Faraday
rotation. This is based on the fact that Faraday rotation of the plane of
polarisation of a radio wave is proportional to the component of gyro
frequency of the electrons along the ray path, which is dependent on the
geomagnetic field as mentioned by Poletti-Liuzzi et al [1976]. Since this
field decreases rapidly with height and is negligible at heights greater than ~
5000 km, the TEC deduced by Faraday rotation method can be assumed to
be obtained upto this height only as shown by Davies[1989]. Now, with the
Differential Doppler method, the integrated electron content upto the satellite
orbit height can be measured, and hence a difference between these two
gives the PEC. As the plasmaspheric content responds directly to solar wind
characteristics and in turn to solar activity, this becomes an important
measurement towards space weather studies.
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Chapter-5
113
are fed to the respective UHF and VHF power amplifiers. This is then fed to
corresponding VHF and UHF Quadra loop antennas. The beacon is powered by an
integrated DC/DC converter, which works from a variable DC input voltage of 28 –
42 V, corresponding to the satellite bus voltages.
The antenna is simple, aerodynamically structured, rugged shorted transmission line
type, consisting of a shorting stub (the radiator), a horizontal conductor and its
reflector or the ground, as shown in figure 5.2. The open end is used to tune the
antenna over a band of frequency proportional to the length of the radiating element
which can be approximated to quarter wavelength of the resonant frequency. As the
frequencies are set wide apart, two antennae, one for VHF and other for UHF are
developed.
Figure 5.2 Schematic diagram of onboard antenna
It is of practical concern that due to the onboard constraint for weight and power
reduction, the output power for the transmitter frequencies are 250 mW for the
carriers and 125 mW for the sub-carriers. The entire system was designed and
developed by AVN, VSSC, with the design details as per preliminary design report
numbered AVN/CRABEX/G-SAT-II/VSSC/1/2K.
5.5.1 Specifications of the beacon transmitter
The specifications of the beacon transmitter are as given in Table 5.1.
Chapter-5
114
Table 5.1 Transmitter specifications
(VHFC) Carrier 150. 01200MHz/250mW
(VHFLSB) LSB 149.01192MHz/125mW
(UHFC) Carrier 400.03200MHz/250mW
(UHFLSB ) LSB 399.03192MHz/125mW
Frequency stability Better than 5 ppm
Short term stability 1 x 10-9 /sec
DC Input Voltage +42V ± 2V
Command Input ON/OFF for DC Input and LSB switch off
Antenna
Type Quarter wave Transmission Line
Centre frequencies 150MHz (VHF) and 400MHz (UHF)
Gain +3.5dBi
Polarisation Linear
VSWR 1.2
5.6 Requirements of a ground receiver system
The ground receiver system for reception of the GSAT-II CRABEX payload
consists of the three major subsystems, Antenna, Signal Processing unit and PC
based data acquisition unit. The design of each subsystem is initiated by finalising
its requirements, which is detailed below.
5.6.1 Antenna subsystem
The antenna should have narrow beamwidth and high gain. The mechanical
structure of antenna has to be moved along the elevation axis, so that the antenna
can be pointed to the exact direction of GSAT-2 after its launch. Once this is fixed,
the system is locked to prevent any further movement. The azimuth of the satellite
plane is measured from True North (geodetic north). It is defined as the direction
along the earth's surface towards the geographic North Pole and differs from the
magnetic north pointed by a compass. True North is fixed using a gyroscope. A
single antenna structure should be able to handle both polarisation receptions as this
Chapter-5
115
will ensure equal phase for the radio waves at the point of reception at the antenna.
Two separate antennae are required for VHF and UHF.
5.6.2 Signal processing subsystem
The receiver system involves reception of very low level signals from the antenna
and hence it is always advantageous to provide a LNA and/or outdoor unit as close
to the antenna as possible to minimize the losses. The cable lengths from the antenna
to the front end should be phase matched at all the frequencies. As the received
signal levels are low, a small phase mismatch due to cable properties can affect the
data quality. To reduce the changes in cable properties, the cables are shielded again
and taken through a small trench from the outdoor unit to the indoor unit. This helps
to ensure normal day-night temperature fluctuations do not affect the signals. The
receiver design should be coherent, so that any strong extraneous noise in the bands
of interest also will not make the system unlock.
5.6.3 DAQ subsystem
The DAQ system has to sample all the analog channels obtained from the receiver
system simultaneously and track them continuously at the user defined sampling
rate. The data acquisition software should start acquisition automatically as soon as
the system locks to carrier frequencies and continuously record eight channel data,
with preferably an online display of the signals being acquired. The data file should
have a header line indicating the sampling rate and the channel name and the
filename should have the date and time of start of acquisition embedded in it. The
DAQ software has to work in Auto mode and Manual mode: in auto mode, the
software automatically records the data onto a new file at 0000 hrs every day,
whereas in manual mode, the file save time is decided by the user.
5.7 Link budget
The link budget for the GSAT system is calculated before initiating the final design
of the receiver system and is given in Table 5.2 below. It is seen that as the
transmitter power is very low, a high gain antenna along with a highly sensitive
receiver system is needed to record the received data.
Chapter-5
116
Table 5.2 Link budget for CRABEX onboard GSAT-II
Frequency (MHz) 150 149 400 399
Tx Power O/P (mw) 250 (24Bm)
125 (21dBm)
250 (24dBm)
125 (21dBm)
Tx Antenna Gain (dBc) +3.0 +3.0 +3.0 +3.0
EIRP (dBm) +27.0 +25.0 +27.0 +25.0
Free Space Loss (dB) -167 -167 -176 -176
Rx Antenna Gain (dB) +18 +18 +18 +18
Polarization Loss (dB) -3 -3 -3 -3
Link Margin (dB) -2 -2 -2 -2
Signal Power at Rx I/p (dBm) -127.0 -130.0 -136.0 -139.0
Rx Noise Temp (°K) 1000 1000 1000 1000
Antenna Noise Temp (°K) 3000 3000 1000 1000
System Noise Temp (dB) 36.02 36.02 33.01 33.01
Boltzmannconstant (dBm/Hz/°K)
-198.6 -198.6 -198.6 -198.6
Rx Noise (dBm/Hz) -162.6 -162.6 -165.6 -165.6
Rx Bandwidth (dBHz) (50Hz) 17 17 17 17
Rx Noise Power (dBm) -145.6 -145.6 -148.6 -148.6
S/N Ratio (dB) 18.6 15.6 12.6 9.6
It is planned to sample the data at 100 samples per second and then do coherent
averaging of 50 samples to get a data value every 0.5 second. This would yield a
further SNR of 17 dB, which is sufficient to obtain an accuracy of 3° in TEC
calculations, and which is acceptable scientifically. Also, a SNR of at least 15 dB
would be essential for studying scintillations because the signal fluctuations at the
magnetic equator could be very high during high solar activity periods. It is seen
from the above table that the weakest signal strength is for 399 MHz. The signal
strength of reference channel of 400 MHz is the crucial one, as this has to make the
receiver lock, so that other data signal can be compared against this to generate the
required phase outputs. Also it can be seen that both the receiving antennas are to
Chapter-5
117
have a high gain of 18dB so as to meet the SNR requirements. A parabolic dish and/
or a Yagi antenna are the possible choices for high gain at these frequencies.
5.8 Receiver system design
The design of the receiver system is highly complex as it has to receive very small
signal levels and also need to continuously monitor and record the data. The
following section highlights the design, development and implementation details of
the major subsystems for the receiver, which consists of antennae, outdoor unit,
indoor unit and a PC based data acquisition unit. The block schematic of this is
shown in figure 5.3.
Figure 5.3 Block schematic of ground receiver system
The ground receiver consists of three antennae immediately followed by three
LNAs. The 150 MHz signal is received at the antenna as left circularly polarised
(LCP, ordinary) and right circularly polarised (RCP, extra-ordinary) component and
fed to the dual 150 MHz channels. For 400 MHz received signal, no separation of
the polarisation components is made as the Faraday rotation measurements at UHF
Chapter-5
118
is small. The outputs from the LNAs are then taken to a single outdoor unit
consisting of amplifiers, mixers and filters for all the channels, and which performs
the down conversion to intermediate frequencies of 10.7 MHz, 9.7 MHz, 4.0125
MHz and 3.0125 MHz. This is taken by two long cables to the indoor unit, where
the phase information is extracted from the received signals using Phase Locked
Loops. The UHF carrier and sideband are locked to two PLLs and are taken as the
reference signals. The VHF carrier and sideband are then phase compared with the
corresponding references and the outputs are taken as quadrature signals. For the
measurement of Faraday rotation at VHF, the output from the LCP and RCP of the
antennae are processed separately and compared in another phase detector. Thus
there are four pairs of outputs from the indoor unit. These are
Phase difference between UHFC and VHFC
Phase difference between VHFS and UHFS
Phase difference between VHFLCP and VHFRCP
Amplitude of UHFC and VHFC
These four pairs of data signals are taken through an 8 channel data acquisition card
to PC where a LabVIEW software performs the data acquisition and archival.
5.8.1 Antenna
The first major component of the receiver system is the antenna. Two different types
of antennae were proposed for GSAT beacon reception. The linearly transmitted
signal from the satellite beacon gets polarised because of its propagation through the
ionosphere, and hence in order to study the ionospheric effects at these frequencies,
polarisation reception antennas are preferred at the ground stations as mentioned by
Evans and Cott [1976]. If we have two separate antennae which can receive these
polarized signals separately, it is possible to process these to find out the rotation
undergone by the radio wave. This in turn is proportional to the Faraday rotation as
detailed by George Kennedy [1977], and this can be used to find TEC as detailed in
Chapter 3.
Thus, in the present context, the antennae proposed are
Chapter-5
119
(i) Crossed Yagi array
This requires two separate antennas, one for VHF (149 and 150 MHz) and other for
UHF (399 and 400 MHz). Each of the Yagi elements are of crossed type so as to
receive both the polarisations at the required frequencies. Though it is needed to
separate out LCP and RCP at VHF only, for design compatibility, similar type of
elements are first planned for both. In such a case, a phase matched network is
needed at UHF antenna output, before LNA block. Both the antenna are mounted
parallel along a single boom, so as to maintain phase coherency and reduce pointing
inaccuracies.
(ii) Parabolic dish antenna
Two separate dish antennae are needed, with one having VHF (149 and 150 MHz)
feed and the other having UHF (399 and 400 MHz) feed. The feed is designed to
receive both polarisations separately. Care should be taken in maintaining pointing
accuracies, as two separate structures are required.
A detailed study is done for optimizing the antenna to be selected for this project. It
is seen that a parabolic dish antenna has higher gain and lower beamwidth than a
crossed Yagi. But at the frequencies of reception here, design of a proper feed poses
a challenge for the same dish size. This can be overcome if the dish sizes are made
in proportion to the frequencies of reception. ie, VHF should have a bigger dish size,
which poses an implementation problem. Finally, this resulted in the selection of
antennae as crossed Yagi for VHF and parabolic dish for UHF. The design and
implementation of both these antennas are detailed below.
5.8.1.1 Specifications of Yagi antenna for VHF reception
The following are the finalised specifications for receiving VHF signals using Yagi
antenna.
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120
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Chapter-5
121
Table 5.4 Yagi antenna design parameters for 150MHz
Nomenclature Length (mm) Spacing
R 1075 0
Dp 975 300
D1 965 429
D2 885 671
D3 875 1008
D4 865 1423
D5 859 1907
D6 857 2451
D7 851 3041
D8 845 3678
D9 845 4346
D10 845 5048
D11 841 5774
D12 839 6504
D13 839 7238
Diameter of Boom 75mm
Diameter of element 152 mm
Length of boom 7.2 mtrs
As it is not possible to measure the antenna pattern accurately onsite after
installation, theoretical evaluation is opted here. However, the return loss of the
antenna was measured after installation and each polarization checked separately.
5.8.1.2 Specifications of parabolic dish antenna for UHF reception
The UHF antenna is designed to receive Right Circular Polarized signals (RCP).
Hence the feed is left circularly polarized, as explained by Constantine Balanis
[1982]. A parabolic Aluminium reflector of 24 metre aperture diameter was
refurbished and installed at SPL (TERLS area) for reception of UHF signals. The
Aluminium reflector is chosen as it is non-corrosive and is suitable for use at humid
Chapter-5
122
places. The feed for the dish antenna is the 400 MHz microstrip patch antenna
presently used for LEOs reception.
The air dielectric microstrip antenna is designed to generate circular polarization
without external polarizer arrangements and is found to be the best choice at this
frequency. This type of antenna is mechanically simple, light weight, less complex,
offers less aperture blockage, gives less phase centre errors and can be designed to
meet required amplitude taper requirements with less complex design.
An almost square patch antenna diagonally fed with a single coaxial feed and having
almost the same dimensions as the one used for LEOS reception and given in Table
4.4 is chosen as feed. This antenna has been designed using 3 mm thick Aluminium
sheet for the ground plane and single side copper-clad Hylam sheet of 1.6 mm thick
for the patch. The copper coated side is kept facing down at a height of 10 mm from
ground plane by using Nylon spacers at the four corners of the top plate. The feed
geometry is shown in figure 5.5.
Figure 5.5 Geometry of 400 MHz feed
Chapter-5
123
The elevation plane patterns showed that the gain of the antenna at 65° from the
zenith falls by about 13.5 dB. Assuming an aperture efficiency of 76 %, the
expected gain at 400 MHz for a parabolic reflector of aperture diameter 20 feet (6.1
meter) with the microstrip antenna as feed is given by
GD= (πd/λ0)2* 76% (5.1)
which expressed in dB is
GD dB= 20*log 10 (πd/λ0) -1.2dB =26.9dB (5.2)
where λ0 is the free space wavelength of 400 MHz frequency and D the aperture
diameter of the parabolic reflector.
The detailed specification of the parabolic dish is given in Table 5.5 below.
Table 5.5 Specifications of parabolic antenna
Frequency of operation 400 and 399 MHz
Gain > 18 dBi
VSWR 1.5 nominal
Impedance 50 Ω coaxial
F/D of antenna 0.4 to 0.5
Polarization of feed LCP
A photograph of the feed realized for 400 MHz is shown in figure 5.6.
Figure 5.6 Photo of 400 MHz feed
Chapter-5
124
5.8.2 Outdoor unit
The outdoor unit is mounted at the base of the antenna structure itself and is
hermetically sealed. The signals from the antennae are routed to the outdoor unit
using phase-matched cables as described in Chapter 4. The specifications for the
outdoor unit is given in the table below and block diagram is shown in figure 5.7.
Table 5.6 Specifications of outdoor unit
No. of input ports 3
Input
RCP 1 VHFC, VHF LSB
RCP 2 UHFC, UHFLSB
LCP VHFC, VHFLSB
No. of output ports 2 (RCP & LCP)
Output
RCP 10.7MHz, 9.7MHz, 4.0125MHz,
3.0125MHz
LCP 4.0125MHz
Gain Better than 30 dB for all frequencies
Noise figure 3dB maximum
Bandwidth 2MHz ± 10%
Input and output impedance 50Ω
VSWR 1.5:1
Max. input handling capability +10dBm
Input signal dynamic range 20dB for all channels.
Input signal level -120 to -140dBm
The outdoor unit comprises of LNAs, preamplifiers, Voltage Controlled Crystal
Oscillator (VCXO), frequency multipliers, mixers, amplifiers, power splitters and
power combiners. There are three channels corresponding to UHF, VHFLCP and
VHFRCP. All channels have similar design architecture.
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125
Figure 5.7 Block schematic of GSAT outdoor unit
The front end of each channel is a LNA. Its main function is to amplify extremely
low signals without adding extra noise, thus preserving the required signal to noise
ratio (SNR) of the systems at extremely low power levels. A good LNA has high
gain, low noise figure, good input and output matching and stability at the lowest
possible current drawn from the amplifier as detailed by Lucek and Damen [1999].
The LNA is followed by preamplifier-filter assembly which gives a linear gain of
~30 dB. For the next stage of down-conversion to IF frequencies, the Local
Oscillator (LO) frequencies, 389.332 MHz and 145.9995 MHz, are generated by
multiplying a stable VCXO, of fundamental frequency 48.6665 MHz by 8 and 3.
The LO1 frequency for the VHF chain is 145.9995 MHz and for the UHF chain is
389.332 MHz. As the onboard frequency has a stability of only 5 ppm, the ground
system is designed to take care of this drift in the frequencies by using a VCXO.
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126
The same VCXO is used for converting the 150 MHz signals of both RCP and LCP
channels to obtain the 4.0125 MHz IF so that the difference between RCP and LCP
components is preserved in phase with respect to the receiver input. After down
conversion, the IF frequencies are VHFC : 4.0125 MHz, VHFLSB : 3.0125 MHz,
UHFC : 10.7 MHz and UHFLSB : 9.7 MHz respectively. The RCP and LCP signals
maintain two separate paths throughout the outdoor unit. The bandwidth of all the
post converter output is 2MHz. The coherent oscillators used for the frequency
conversion to IF ensure the phase relationship between the input frequencies and the
output frequencies. The power supply and the VCXO signal for this unit come from
the indoor unit through the output ports (RCP & LCP). The RCP signals are
combined at the output of the outdoor unit and brought out through a single
connector. This helps reduce the number of long cables to the indoor unit, as well as
to maintain the signal integrity.
5.8.3 Indoor unit
There are two input ports for this unit. The indoor unit comprises of power splitters,